In oauth_clients_controller.rb, find and replace the line reads
@tokens = current_user.tokens.find(:all, :conditions => 'oauth_tokens.invalidated_at is null and oauth_tokens.authorized_at is not null')

with

@tokens = current_user.tokens.includes(:client_application).where('oauth_tokens.invalidated_at is null and oauth_tokens.authorized_at is not null')

In lib/oauth/rack/oauth_filter.rb of oauth-plugin gem, find the line (around line #27) which reads:
if token = Oauth2Token.first(:conditions => ['invalidated_at IS NULL AND authorized_at IS NOT NULL and token = ?', token_string])